Telescoped multiwall nanotube and manufacture thereof

ABSTRACT

The invention relates to a method for forming a telescoped multiwall nanotube. Such a telescoped multiwall nanotube may find use as a linear or rotational bearing in microelectromechanical systems or may find use as a constant force nanospring. In the method of the invention, a multiwall nanotube is affixed to a solid, conducting substrate at one end. The tip of the free end of the multiwall nanotube is then removed, revealing the intact end of the inner wall. A nanomanipulator is then attached to the intact end, and the intact, core segments of the multiwall nanotube are partially extracted, thereby telescoping out a segment of nanotube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. Ser. No.09/915,196, now U.S. Pat. No. 6,874,668 which claims priority under 35U.S.C. §119(e)(1) to U.S. Provisional Patent Application Ser. No.60/220,550, filed Jul. 25, 2000, both of which are hereby incorporatedby reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for forming a low-friction,nanoscale bearings from a multiwall nanotube. The invention findsutility in the field of microelectromechanical systems (MEMS)technology.

BACKGROUND

There is currently substantial interest in reducing the size scale ofvarious mechanical systems, and dramatic recent advances have beenachieved in the fabrication of microelectromechanical systems. A reviewof current trends in micromelectromechanical systems can be found in W.Trimmer, (1997) Micromechanics and MEMS: Classic and Seminal Papers to1990, IEEE Press, New York. Ultimately, such systems are expected toreach well into the nanometer domain, and hence considerations of themechanical behavior of materials at the nano- or atomic-scale, includingthose related to atomic perfection and friction, become criticallyimportant.

Carbon nanotubes, as discussed in Iijima (1991) Nature 354:56-58, haveunique mechanical and electronic properties that make them outstandingcandidates for nanomechanical or nanoelectromechanical applications,such as nanoscale electronics, see Collins et al. (1997) Science278:100-103, and nanoscale mechanical systems, see Iijima (1998) Proc.IEEE Eleventh annual International Workshop on Micro Elector MechanicalSystems (IEEE, Heidelberg, Germany), 520-525. For a discussion of themechanical properties of carbon nanotubes see Iijima et al. (1996) J.Chem. Phys. 104:2089-92, Poncharal et al. (1999) Science 283:1513-16,and Wagner, et al. (1998) Appl. Phys. Let. 72:188-90. For a review ofthe electronic properties of carbon nanotubes see N. Hamada et al.(1992) Phys. Rev. Lett. 68:1579-81, and Saito et al. (1992) App. Phys.Let. 60:2204-6.

Multiwall carbon nanotubes (MWCNTs) comprise concentric cylindricallayers or shells of graphite-like sp²-bonded carbon, where theintershell interaction is predominantly van der Waals. In analogy to thewell-known lubricating properties of van der Waals bonded graphite, theindividual cylinders of MWNTs might be expected to easily slide orrotate with respect to one another, forming near-ideal linear androtational nanobearings. Recent theoretical calculations disclosed byKolmogorov et al. ((2000) Bulletin of the APS, March Meeting 2000,Minneapolis, Minn. (American Institute of Physics)) and Crespi etal.((1999) in Electronic Properties of Novel Materials—Science andTechnology of Molecular Nanostructures, Kuzmany et al., Eds. (AmericanInstitute of Physics, College Park, Md. pp. 364-368) indicate that theMWNT interlayer corrugation energy is indeed small, favoring suchmotion. For a MWNT, one could envision an extension mode much like the“telescoping” of a mariner's traditional spyglass. Some evidence forinadvertent MWNT telescopic extension can be found in severe mechanicalstress failure mode studies, including MWNTs embedded in a stressedpolymer composite, see Wagner et al. (1998) Appl. Phys. Let. 72:188-90,and for MWNTs torn apart in quasi-static tensile stress measurementsperformed in a scanning electron microscope, see Yu et al. (2000)Science 287:637-640. However, no demonstration of controlled andreversible telescoping of MWNTs has been previously achieved.

A major difficulty in initiating controlled telescoping in MWNTs is thecommonly capped ends that seal in all inner core nanotube cylinders.Even if the MWNT ends are opened by methods such as acid etching, it isdifficult to selectively contact only the core tubes. Recently, a methodhas been disclosed whereby the ends of a MWNT can be carefully opened,removing the caps from just the outer shell nanotubes while leaving thecore nanotubes fully intact and protruding, see Cumings et al. (2000)Nature 406:586. It has been found that the method of Cumings et al. canbe used to attach a moveable nanomanipulator to the core nanotubeswithin a MWNT. This attachment allows for in-situ manipulation of thenanotube core thereby providing controlled reversible telescoping.Robust ultra-low friction linear nanobearings and (constant-force)nanosprings are demonstrated.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome theabove-mentioned disadvantages of the prior art by providing a method forforming a telescoped multiwall nanotube and a device comprising atelescoped multiwall nanotube that may find utility as a low-frictionbearing or constant force spring.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description that follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned through routine experimentation uponpractice of the invention.

In one embodiment, the invention relates to a method for forming atelescoped nanotube. First, a multiwall nanotube is provided. Thenanotube is comprised of an outer shell, a plurality of concentric innershells, and an inner core. Each of the plurality of concentric innershells, the outer shell, and the inner core has a first end an anopposing second end, and the first end of the outer shell is attached toa conducting substrate so as to be in electrical communicationtherewith. Next, the second ends of the outer shell and the concentricinner shells are removed, revealing the second end of the inner core. Ananomanipulator is then attached to the exposed second end of the innercore and the inner core is partially extracted from the outermost shell,thereby telescoping one segment of the multiwall nanotube to provide thetelescoped multiwall nanotube. The nanomanipulator is then optionallydetached from the inner core.

Optionally, the inner core is comprised of secondary concentric innershells and a secondary inner core, each having first and second ends,the end removal, nanomanipulator attaching, and core extracting stepsare repeated so that multiple segments of nanotube are sequentiallytelescoped. Generally, at least three segments of nanotube aretelescoped. The opposing end of the outer shell is removed using ashaping electrode and is carried out while the nanotube and the shapingelectrode are under a potential difference.

In another embodiment, the invention pertains to a device comprising atelescoped multiwall nanotube. The device is comprised of an outer shellhaving a cylindrical wall, a closed end, and an interior cavity definedby the cylindrical wall and the closed first end. A telescoped segmentis partially housed within the interior cavity of the outer shell andpartially extends from the outer shell, the segment having a cylindricalsegment wall, a closed segment end, and a segment cavity. The telescopedsegment may comprise a plurality of concentric telescoped segments. Eachconcentric segment is partially housed within the segment cavity of theconcentric segment surrounding it and each segment partially extendsfrom the segment cavity of the segment in which it is housed. Suchconcentric telescoped segments are generally movable and may provide astatic spring force. Such concentric telescoped segments may alsoprovide substantially frictionless rotation, extension, and retraction.

The telescoped multiwall nanotube is comprised of a material selectedfrom the group consisting of: GaSe; NiCl₂; TiO₂; Sb₂S₃; K₄Nb₆O₁₇;PbNb_(m)S_((2m+1)), where m is an integer from 1 to 10; B_(x)C_(y)N_(z),where x is about 0 to about 1, y is about 0 to about 3, and z is about 0to about 4; MX_(n) where M is selected from the group consisting of Nb,V, Zr, Hf, Re, Pt, Ta, W, and Mo, X is selected from the groupconsisting of S, Se, and Te, and is n is 2 or 3; and W_(a)Mo_(b)C_(c)S₂wherein a is about 0 to about 3, b is about 0 to about 3, and c is about0 to about 4. Preferably the material is carbon. The concentrictelescoped segments may be comprised of a plurality of concentric innershells range from about 3 to about 1000. Ranges of from about 3 to about100 are more customary and ranges from about 3 to about 50 arepreferred.

Such a telescoped multiwall nanotube may find use as linear orrotational bearing in a MEMS or may find use as a constant forcenanospring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(Amended))-1(f), collectively referred to as FIG. 1 schematicallyillustrate the method of the inventions and several manipulations madeof the telescoped multiwall nanotube so produced.

FIG. 2 shows a high-resolution transmission electron microscope (TEM)image of a multiwall nanotube in a fully telescoped position.

FIG. 3 shows several selected frames from a video recording of theaccelerated motion of a core segment after release from thenanomanipulator.

FIG. 4( a) shows a close-up TEM image of a bamboo joint inside a largermultiwall nanotube. In FIG. 4( b), the core segment on the right hasbeen telescoped out, cleanly separating the bamboo joint.

DETAILED DESCRIPTION OF THE INVENTION

Before the invention is described in detail, it is to be understood thatunless otherwise indicated this invention is not limited to particularmaterials, components or manufacturing processes, as such may vary. Itis also to be understood that the terminology used herein is forpurposes of describing particular embodiments only, and is not intendedto be limiting.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a nanotube” includes a single nanotube as well as two ormore nanotubes, “a shaping electrode” includes a single shapingelectrode as well as an assembly of two or more shaping electrodes, “alayer” may be a single layer or may comprise two or more layers and thelike.

In this specification and in the claims that follow, reference will bemade to a number of terms that shall be defined to have the followingmeanings unless the context clearly indicates otherwise:

The term “nanotube” as used herein refers to a solid, cylindricallyshaped and discrete fibril typically characterized by a substantiallyconstant diameter of typically about 1 nm to about 100 nm, preferablyabout 2 nm to about 50 nm. In addition, the nanotube typically exhibitsa length greater than about 10 times the diameter, preferably greaterthan about 100 times the diameter. The term “multiwall” as used todescribe nanotubes refers to nanotubes having a layered structure, suchas a fullerene-like structure, so that the nanotube comprises an outerregion of multiple continuous layers of ordered atoms and an optionaldistinct inner core region or lumen. The layers are disposedsubstantially concentrically about the cylindrical axis of the fibril.

The term “transition metal chalcogenide” is used herein in its ordinarysense and refers to a compound having at least one metallic elementhaving an incomplete inner electron shell, marked by multiple valencesand at least one element from Group 16 of the period table. As usedherein all reference to the elements and groups of the Periodic Table ofthe Elements is to the version of the table published by the Handbook ofChemistry and Physics, CRC Press, 2000, which sets forth the new IUPACsystem for numbering groups. Thus, the term “transition metalchalcogenides” include mixed metal chalcogenides, metal mixedchalcogenides and mixed metal mixed chalcogenides. Preferred transitionmetals include titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromiuim, molybdenum, tungsten, iron, ruthenium, osmium,cobalt, nickel, copper and silver. Preferred chalcogens include sulfur,selenium and tellurium.

The term “potential” is used herein in its ordinary sense and describesan electrostatic phenomenon as a scalar quantity. Thus, the term“potential difference” is used in its ordinary sense to refer to thechange in potential energy by a test charge. Similarly, the term“ground” refers to the potential of an effectively infinite sink towhich electrons can easily migrate.

Thus, one embodiment of the invention relates to a method for forming atelescoped nanotube. First, a multiwall nanotube is provided. Multiwallnanotubes are well known, as are methods of constructing them. The mostcommon and easily constructed nanotubes are made from carbon usingcarbon arc discharge methods. Techniques for producing carbon nanotubesare well known in the art. Carbon nanotubes are widely commerciallyavailable and may be obtained, for example, from Hyperion CatalysisInternational, Inc. (Cambridge, Mass.) or from Carbon NanotechnologiesIncorporated (Houston, Tex.). See Iijima (1991) Nature 354:56-58. Othermethods and materials are also known in the art. For example, U.S. Pat.No. 6,231,980 to Cohen et al., discusses nanoscale particles and tubesmade from a variety of stoichiometries of B_(x)C_(y)N_(z) where x, y,and z indicate a relative amount of each element compared to the othersand U.S. Pat. No. 5,958,358 to Tenne et al. discloses a method ofpreparing polycrystalline nested nanotubes of transition metalchalcogenides.

The exact method and construct of the multiwall nanotube is not criticaland a suitable multiwall nanotube can be comprised of any material solong as the material is layered in nature, i.e., having a structurewherein certain atoms of the structure are held together by stronglydirected covalent or ionic bonds in a hexagonal or other array to formlayers, while bonds transverse to the layers are formed as a result ofweak forces such as van der Waal forces. Examples of suitable materialsinclude, but are not limited to, carbon (e.g., graphite), boron nitride,boron carbon nitride and transition metal dichalcogenides. Materialshaving a crystalline structure selected from the group consisting ofGaSe, NiCl₂;, TiO₂, Sb₂S₃, K₄Nb₆O₁₇, and PbNb_(m)S_((2m+1)), where m isan integer from 1 to 10, are all suitable. Also suitable are materialshaving the crystalline structure B_(x)C_(y)N_(z), where x is about 0 toabout 1, y is about 0 to about 3, and z is about 0 to about 4. Suitablechalcogenides and dichalogenides include, but are not limited to,compounds having the formula MX_(n) where M is selected from the groupconsisting of Nb, V, Zr, Hf, Re, Pt, Ta, W, and Mo, X is selected fromthe group consisting of S, Se, and Te, and is n is 2 or 3. Othersuitable materials are those having the structure W_(a)Mo_(b)C_(c)S₂wherein a is about 0 to about 3, b is about 0 to about 3, and c is about0 to about 4. Optionally, these materials may be doped with traceamounts of any of a number of elements to improve certain properties ofthe materials for a particular purpose, e.g., to decrease the electronicband gap to enhance electrical conductivity. Regardless of the materialselected, the multiwall nanotube is comprised of an outermost shell, aplurality of concentric inner shells, and an inner core, each havingfirst and second opposing ends.

Once a suitable nanotube is provided, a first end of the nanotube isattached to a suitable conducting substrate. Both the substrate andmaterials employed to attach the nanotube to the substrate should bemade from a material that does not represent a source of contaminationduring the shaping process. Thus, the substrate may be formed from aninert material such as gold, platinum or other noble metal and theattachment may be made via a solid carbonaceous material deposited on tothe surface of the conducting substrate. Such methods are described inYu et al. (2000) Science 287:637-640. As discussed in Yu et al., thecarbonaceous material is deposited by focusing an electron beam at thedesired location on the substrate. The electron beam disassociatesresidual organic species on the substrate surface, which are thendeposited onto the substrate surface. The rate at which the carbonaceousmaterial is deposited on the substrate is dependent upon the imagingparameters, the vacuum quality, and surface contamination on thesubstrate. Additionally, organic contamination on the substrate surfacemay also migrate toward the focal point of the electron beam and bedissociated and deposited.

Other suitable methods of attaching the nanotube to the substrateinclude employing adhesives known in the art such as acrylates,methacrylates or epoxies (in liquid, tape of other forms); employingelectrostatic forces; spot welding; mechanical clamping or fixing;exploiting the chemical affinity between the substrate and the nanotube;or synthesizing the nanotube on the substrate. In addition, such methodsmay be employed in combination. For example, during nanotube synthesis,a number of other phases may also be synthesized from an aggregate thatencapsulates the nanotube. Such an aggregate may be immobilized withrespect to a substrate using a conductive adhesive. Then, mechanicalenergy may be applied to the aggregate to fracture the aggregate. As aresult, the nanotube is exposed.

After attachment of the nanotube to the conducting substrate, the secondends of the outer shell and inner concentric shells are removed therebyexposing the second end of the inner core. The removal of these ends isaccomplished by the use of a shaping electrode. The material removal iscarried out while the nanotube and the shaping electrode are under apotential difference. Typically, the potential difference is no morethan about 10 volts. However, the potential difference is preferably isno more than about 5 volts and optimally about 0.5 to about 3.0 volts.In addition, it is preferred that the potential of the nanotube is at ornear ground. The use of such a shaping electrode is disclosed incopending and commonly assigned U.S. application Ser. No. 09/915,207,entitled “A METHOD SHAPING FOR A NANOTUBE AND A NANOTUBE SHAPEDTHEREBY,” filed Jul. 24, 2001. While any of a number of shapingelectrodes may be employed, the preferred shaping electrode comprises anadditional nanotube located at the end of a nanomanipulator. Thenanotube may be of the same material as the multiwall nanotube to betelescoped, or may comprise a different material. If that nanotube isused as the shaping electrode, the nanotube so used will typically belarger than the nanotube to be shaped.

Depending on the potential difference, removal of the ends may takeplace when the shaping electrode contacts the second end of the outershell or when the shaping electrode is sufficiently close to the secondend of the outer shell. Thus, removal of the second ends of the outershell and the inner concentric shells may not require contact betweenthe shaping electrode and the nanotube.

In some instances, material removal at the second ends of the outershell and inner concentric shells can be carried out by first placingthe shaping electrode in contact with the nanotube when the shapingelectrode and the nanotube do not exhibit a sufficient potentialdifference for material removal from the nanotube and then controllablyincreasing the potential difference between the electrode and thenanotube to remove the second ends of the outer shell and the innerconcentric shells. In other instances, the shaping electrode may beplaced in a non-contacting yet proximal relationship with the nanotubewhereby the shaping electrode and the nanotube do not exhibit asufficient potential difference for material removal. Once in place, thepotential difference between the electrode and the nanotube is thencontrollably increased, thereby removing the second ends of the outershell and inner concentric shells and exposing the second end of theinner core. Preferably, material removal is carried out under vacuum,since gases that are ordinarily considered inert may react with thenanotube.

As another option, a constant potential difference may be generatedbetween the nanotube and the shaping electrode when they are not inproximal relationship with each other. Then, under the constantpotential difference, the nanotube and the shaping electrode are broughtinto a proximal or contacting relationship effective to remove thematerial from the end of the multiwall nanotube. Thus, modification ofboth the relative distance and potential difference between themultiwall nanotube and the shaping electrode may be used to control theremoval of material from the multiwall nanotube.

While any of a number of shaping electrodes may be employed, thepreferred shaping electrode comprises an additional nanotube. Theadditional nanotube may comprise the same material as the multiwallnanotube or may be different. Typically, the additional nanotube islarger than the multiwall nanotube to be telescoped.

A nanomanipulator is attached to the exposed second end of the innercore via spot welding or other suitable method and the inner corepartially extracted from the outermost shell, thereby telescoping onesegment of nanotube. Once the segment is telescoped, the nanomanipulatoris removed. The nanomanipulator is generally an atomic force microscopeprobe, although other suitable nanomanipulators may be used, and arecommercially available. Any of a number of nanomanipulators (also termed“nanopositioners”) such as those used in electron microscopy may beemployed or adapted for the present invention. Nanomanipulators arecommercially available and may be obtained, for example, from PiezomaxTechnologies, Inc (Middleton, Wis.) or 3rdTEch, Inc. (Chapel Hill,N.C.). Further information regarding the manipulation of nanotubes canbe found in Falvo et al. (1999), “Rolling and sliding of carbonnanotubes,” Nature 397:236-238, Falvo et al. (1998), “Nanomanipulationexperiments exploring frictional and mechanical properties of carbonnanotubes,” Microscopy and Microanalysis 4: 504-512, Falvo et al.(1997), “Bending and buckling of carbon nanotubes under large strain,”Nature 389:582-584, Paulson et al. (1999), “In situ resistancemeasurements of strained carbon nanotubes,” App. Phys. Lett. 75(19):2936-2938.

It will be obvious to one of skill in the art that the nanomanipulatorand the shaping electrode may comprise the same device and the removalof the second end of the outer shell and the attachment of thenanomanipulator may occur simultaneously. For example, if the shapingelectrode directly contacts the multiwall nanotube and removal of thesecond end of the outer shell is effected by the increase of thepotential difference between the shaping electrode and the multiwallnanotube, the attachment of the shaping electrode to the intact secondends of the inner shells may occur as the potential difference betweenthe shaping electrode and the multiwall nanotube decreases.

If desired, the second end of the inner core of the multiwall nanotubemay be removed using the above-described method, thereby exposing thesecond ends of any of secondary concentric inner shells. The newlyexposed second ends may then also be contacted with a nanomanipulatorand telescoped.

FIG. 1 schematically illustrates the steps involved in the inventivemethod and possible manipulations of the telescoped nanotubes. Anas-grown MWNT produced by conventional arc-plasma methods is firstrigidly mounted as shown in FIG. 1( a), and the free end of the MWNT isthen removed to expose the core tubes, as shown in FIG. 1( b). In FIG.1( c), the nanomanipulator is brought into contact with the core tubesand, using electrical current, is spot welded to the core.

FIGS. 1( d), 1(e), and 1(f) show three different manipulations of thetelescoped nanotubes. In FIG. 1( d) the manipulator is moved right andleft, thus telescoping the core out of, or reinserting it into, thehousing of outer shell. The extraction/reinsertion process has beenrepeated numerous times, all the while viewing the MWNT at high TEMresolution to test for atomic-scale nanotube surface wear and fatigue.In FIG. 1( e), the manipulator first telescopes out the inner core andthen fully disengages, allowing the core to be drawn back into the outershells primarily by the inter-tube van der Waals energy-lowering force.In FIG. 1( f), a telescoped nanotube is subjected to additionaltransverse displacements, and reversible mechanical failure modes suchas buckling and complete collapse are induced.

FIG. 2 shows a TEM image of a MWNT in a fully telescoped position. Usinghigher resolution imaging than that used for FIG. 2, it has beendetermined that this MWNT originally had 9 walls, with an outer diameterof 8 nm and an inner diameter of 1.3 nm. After extension, a 4 nmdiameter core segment (consisting of 4 concentric walls) has been almostcompletely extracted from the outer shell structure. The telescopingprocess was found to be fully reversible, in that the core could becompletely pushed back into the outer shells, restoring the MWNT to itsoriginal “retracted” condition. The process of extending and retractingthe core was repeated many times for several different MWNTs, and in allcases no apparent damage to the “sliding” surfaces, i.e., the outer tubeof the core or the inner tube of the shell structure, was observed, evenunder the highest TEM resolution conditions (˜2 Å). The apparent lack ofinduced defects or other structural changes in the nanotube contactsurfaces at the atomic level suggests strongly that these nearatomically perfect nanotube structures may be wear-free and will notfatigue even after a very large number of cycles.

In the engineering of macroscopic bearings, the moving parts aretypically cycled 10³ to 10⁹ times before definitive conclusions aboutwear can be drawn. This is because the damage from a single cycle ismicroscopic and cannot be readily observed by eye or even conventionalmicroscopy. Here, the bearings are observed at the atomic scale, andafter all cycles, from the first cycle onward, the atomic structure ofthe nanotubes is unaffected by motion. Thus, the nanotube sections ofthe invention are near-perfect sliding surfaces and are apparently freefrom wear for all cycles.

It has also been observed that repeated sliding motion for a given MWNTtakes place between the same nanotube shells. While not wishing to bebound by theory, applicants believe that this repeatability is theresult of a self-selection process wherein the most perfect surfacesoffer the least resistance to motion. Further evidence for this beliefis found in the fact that after repeated motions, the same surfacesremained the “most favored.” This fact also provides evidence of thelack of sliding-induced wear on the active surfaces. Of course, in amany-walled MWNT, even the catastrophic failure, i.e., fusing, of onesurface pair would not render the MWNT bearing unusable, as another(nearly equally perfect) surface pair would simply become the activeelements.

Several internal forces are associated with telescoping MWNTs. To firstorder, these are the van der Waals-derived force and possible static anddynamic frictional forces. The van der Waals force is given by

$F_{vdW} = {\frac{\mathbb{d}}{\mathbb{d}x}{U(x)}}$where the van der Waals energy, see Benedict et al. (1998) Chem. Phys.Let. 286:490-6, is given by U(x)=−0.16Cx joules with C as thecircumference of the “active” nanotube bearing cylinders and x as thelength of the overlap between the core section and the outer walls, bothmeasured in meters. The van der Waals energy lowering gained byincreasing the tube-tube contact area tends to retract the extended coreof a telescoped MWNT. Since the active intertube contact area decreaseslinearly with core tube extension, this restoring force is independentof contact area, or equivalently, independent of core extension. Hence,a telescoped nanotube with only one active (sliding) surface pair isexpected to act as a constantforce spring.

To determine experimentally if F_(vdW) dominates nanotube linear bearingdynamics, the configuration described in FIG. 1( e) was used. The coretubes of a MWNT were first telescoped using the manipulator. Lateraldeflections of the manipulator were used to fatigue and eventually breakthe spot weld, thus releasing the core segment. FIG. 3 shows severalselected frames from a video recording of the accelerated motion of areleased core segment. The upper five frames show the core segment beingslowly and successively telescoped to the right (the structure in theleft third of the image seen crossing the MWNT at about a 30° angle isanother nanotube unrelated to the experiment and it is not in physicalcontact with the subject MWNT). Just after the fifth frame themanipulator has released the core segment. The sixth and final frame,which occurred one video frame after the release of the nanotube, showsthe core after it rapidly and fully retracted inside the outer shells ofthe MWNT. The dimensions for the core segment of the MWNT of FIG. 3yield a core segment mass 2.9×10⁻¹⁶ g. Combining this with C=57 nm andthe initial extension of 330 nm, Eq. (1) leads to complete retraction ofthe core tubes in 4.6 ns. This is consistent with our experimentalobservation that the complete contraction occurred in less than onevideo frame (33 msec).

While macroscopic models of friction between solids dictate thatfriction is proportional to normal force, independent of contact area,modern microscopic models of friction predict that friction is in factproportional to contact area, see Persson (1999) Surf. Sci. Rep.33:83-119. In macroscopically rough samples, the actual contact occursat point asperities, and the microscopic contact area is proportional tothe total normal force. Nanotube shells, however, are atomically smooth,so any interlocking between the shells (due, for example, to the atomiccorrugations) is best estimated by using the entire surface area ofcontact. The F_(vdW) retraction force for the nanotube in FIG. 3 iscalculated to be a mere 9 nN. This indicates that the static frictionforce is small, with f_(s)<2.3×10⁻¹⁴ newtons per atom (6.6×10⁻¹⁵ newtonsper Å²). The full contract the tube provides that the dynamic frictionf_(k)<1.5×10⁻¹⁴ newtons per atom (4.3×10⁻¹⁵ newtons per Å²). Friction isan important concern in small-scale systems such as MEMS. Recentatomic-scale frictional force measurements using conventional materialsyield values approximately, three orders of magnitude greater than theupper limit on frictional forces found here for MWNT surfaces.

Minimization of bearing friction may be obtained by using the shortestpossible nanotube core/housing overlap. This may be achieved byminimizing the length of the MWNT, maximizing the extraction length, andor by using a “bamboo” configuration such as that disclosed in Iijima etal. (1992) Phys. Rev. Let. 69:3100-3. In such bamboo configurations, theinner nanotube sections do not extend along the entire length of theMWNT, but rather represent an end-to-end series of shorter, fullycapped, nanotube segments residing inside the continuous housing ofouter nanotube shells. FIG. 4( a) shows a close-up TEM image of such abamboo joint inside a larger MWNT. In FIG. 4( b), the core segment onthe right has been telescoped out, cleanly separating the bamboo joint.Use of bamboo type MWNT has practical implications for common-axisbearings as short independent bamboo sections might serve as ultra lowfriction linear or rotational bearings that are firmly embedded in acommon long, stiff cylindrical housing.

FIG. 1( f) illustrates lateral deformations of partially telescopednanotubes. Upon lateral deformation, telescoped MWNTs form kinks and mayeven fully collapse. MWNTs with large inner diameters and few concentricshells are particularly susceptible to kinking and collapse. MWNT kinkand collapse much more readily after the inner core has been removed.For example, a 40 layer core was telescoped out from a nanotube having60 original layers with an outer diameter of 43 nm to a maximumextension of 150 nm leaving an outer shell housing of just 20 layerswith an inner diamter of 29 nm. The housing was supported at the baseand the inner core section of the tube was still engaged in the housingfor a length of 200 nm. When the manipulator was driven laterally toapproximately ˜5° angular displacement, the housing shells developed akink in the middle of the large inner diameter section. At ˜26°displacement the kink was severe and resembled the schematic in FIG. 1(f). At any displacement angle, the telescoped core section was stillmobile, and could be moved back and forth inside the unkinked portion ofthe outer shell housing. At small kink angles less than ˜10°, the corecould be inserted past the kink position, forcing the kink to disappearthereby reinflating the outer shells to their original circular crosssection. At more severe bending angles, in excess of ˜20°, the kinkblocked the inner core section from being fully inserted. Hence,suitable kinking of the outer shell housing provides an effective motionstop for nanotube core insertion.

The above results demonstrate that MWNTs hold great promise fornanomechanical or nanoelectromechanical systems applications.Low-friction low-wear nanobearings and nanosprings are essentialingredients in general NEMS technologies. The transit time for completenanotube core retraction is on the order of 1-10 nsec, which implies thepossibility of exceptionally fast electromechanical switches. In thetelescoped geometry, a carbon nanotube could also be used as a variableresistor. It is possible to pass electrical current through the innershells of the telescoped nanotube to the outer shell during thetelescoping, and as such the nanotube forms an electrical resistor. Asthe nanotube is telescoped further out, the total length of the nanotubeis increased, and the amount of overlap between the core and the housingis decreased. If the conduction of the nanotube is governed by Ohm's law(as may be the cases for large or highly defective nanotubes), then theresistance of the nanotube should increase linearly with the extensionof the telescoping. If the nanotube is smaller, or of a higher degree ofperfection, the conductivity may be governed by the rules of quantumballistic transport (mesoscopic physics). In such a case, the dependenceof the resistance on length may be linear, constant (independent oflength), or non-linear. The non-linear dependence may be periodic(oscillating on the scale of atomic distances as individual atoms slideout of the overlap region), exponential (governed by the rules oflocalization), or some other dependence not yet theoretically predicted.The case of linear dependence would be the easiest to apply, as it is asimple resistance potentiometer, but the other cases may find even moreutility in future nanoscale applications.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description and examples are intended to illustrate and notlimit the scope of the invention. Other aspects, advantages andmodifications within the scope of the invention will be apparent tothose skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

1. A device comprising a telescoped multiwall nanotube comprised of: (a)an outer shell having a cylindrical wall, a closed end, and an interiorcavity defined by the cylindrical wall and the closed end; and (b) atelescoped segment partially housed within the interior cavity of theouter shell and partially extending from the outermost shell, whereinsaid telescoped segment has a cylindrical segment wall, a closed segmentend, and a segment cavity.
 2. The device of claim 1, wherein thetelescoped segment comprises a plurality of concentric telescopedsegments each partially housed within the segment cavity of theconcentric telescoped segment surrounding it and each partiallyextending from the segment cavity of the telescoped segment in which itis housed.
 3. The device of claim 2, further comprising an innermostconcentric telescoped segment having a closed end on the partiallyextended portion.
 4. The device claim 3, wherein the multiwall nanotubecomprises a material selected from the group consisting of: GaSe; NiCl₂;TiO₂; Sb₂S₃; K₄Nb₆O₁₇; PbNb_(m)S_((2m+1)), where m is an integer from 1to 10; B_(x)C_(y)N_(z), wherein x is about 0 to about 1, y is about 0 toabout 3, and z is about 0 to about 4; MX_(n) where M is selected fromthe group consisting of Nb, V, Zr, Hf, Re, Pt, Ta, W, and Mo, X isselected from the group consisting of S, Se, and Te, and is n is 2 or 3;and W_(a)Mo_(b)C_(c)S2 wherein a is about 0 to about 1, b is about 0 toabout 3, and c is about 0 to about
 4. 5. The device of claim 4, whereinthe material is carbon.
 6. The device of claim 1, wherein the concentrictelescoped segment is comprised of from about 3 to about 1000 concentricinner shells.
 7. The device of claim 6, wherein the concentrictelescoped segment is comprised of from about 3 to about 100 concentricinner shells.
 8. The device of claim 7, wherein the concentrictelescoped segment is comprised of from about 3 to about 50 concentricinner shells.
 9. The device of claim 1, wherein the concentrictelescoped segment is movable.
 10. The device of claim 9, wherein theconcentric telescoped segment provides a static spring force.
 11. Thedevice of claim 9, wherein the concentric telescoped segment providesfor substantially frictionless rotation.
 12. The device of claim 9,wherein the concentric telescoped segment provides for substantiallyfrictionless extension and retraction.
 13. A bearing comprising thedevice of claim
 9. 14. A switch comprising the device of claim
 9. 15. Aresistance potentiometer comprising the device of claim 9.